1. Field of the Invention
The present invention relates to the field of digital video compression and decoding. In particular it enables a compressed video bitstream to be decoded by multiple decoders each of which has a lower decoding frame rate capability than that of the target display frame rate.
2. Related art of the Invention
Most video material today are available in interlaced format at 60 fields per second. Interlaced format introduces many artifacts such as screen flicker that are annoying to the viewer. With current display technology it is now possible to display progressive format at 60 frames per second. This means that the viewer need not have to content with the interlace artifacts. In fact by using progressive format the subjective image quality is improved significantly.
MPEG 2 is the latest international standard for coding digital video. The digital video is compressed into bitstreams that can be decoded by a decoder for display. Using the Main Profile at High Level it is possible to code progressive video at 60 frames per second. The current invention deals with the method for achieving the above decoding method.
Currently decoder chips conforming to the Main Profile at Main Level are available and are able to decode compressed bitstream at up to 30 frames per second. In order to decode at twice the frame rate required by the progressive format a faster decoder have to be designed. On the other hand, the mismatch in the decoding and display frame rate can be solve by using more than one decoder in parallel. However the solution is not as trivial as simply to put two decoders together and dividing up the bitstream among the decoders. The reason is that the compressed bitstream is organized in groups of pictures, GOP, and each GOP contains intra coded pictures, I-pictures, forward predicted pictures, P-pictures, as well as bidirectionally coded pictures, B-pictures. This is depicted in FIG. 1 which shows the organization of the bitstream for the I-, P- and B-pictures in transmission order starting from left to right.
It is not possible to simply assign one GOP to one decoder because the decoding of the first two B-pictures in the GOP requires the reference to the last P-picture of the previous GOP as shown in FIG. 2. Similarly it is not feasible to partition the bitstream at the position indicated by the arrow in FIG. 2 as the P-picture after the arrow requires the reference to the preceding I-picture. Since more than one decoder is used in parallel, and each decoder only receives its share of the bitstream the reference picture will not be available at the boundary where the bitstream is partitioned. If we simply duplicate the bitstream then there will be a duplication of the I-picture and B-pictures at the boundary of the GOP.